Minimize lasting physical damage: if possible "do not damage to the information container" before or after the information is communicated.

Minimize physical evidence of trauma: Physical scars can prove that torture was used, and might reduce admissibility or credibility of the prisoner's confessions.

Maximize your future torture options: You can't use stress positions if the participant's hands have been amputated. You can't show your prisoner his wife being raped if you've burned his eyes out.

Minimize recovery time: the sooner you can get them coherent, the more likely that they'll answer questions. No wasting valuable interrogation time while your subject revives.

Waterboarding is a big winner in all categories. Immersion of the sinuses, pharynx, larynx, and trachea is hardwired into the brain's highest-priority reflexes as a life-threatening situation, inhibiting rational cognition and resistance training in a way that hot pokers, jumper cables, and bamboo splinters can't. Waterboarding doesn't leave lasting marks either; as soon as the participant is toweled off there's only the word of a dirty terrorist to prove that anything happened. Because the lungs are elevated, the risk of actual drowning from simulated drowning is minimized. If asphyxiation *does* occur, the doctors who stand by to advise how much more waterboarding their patient's physiology can endure have an extremely generous window of five to six minutes to revive him with oxygen before brain damage starts.
Waterboarding is "torture plus". The minimal evidence of physical trauma, rapid recovery time, repeatability, and relatively low lethality do not make waterboarding "less severe" or more humane, they are the features that make it a convenient technique for captors to create unbearable suffering with minimal evidence.